PROJECT SUMMARY Metabolic liver disease is an emerging public health problem. In the United States, diabetes- and obesity-related metabolic liver disease is the most common cause for orthotopic liver transplantation (OLT), which requires life-long immunosuppression and is associated with substantial morbidity and mortality (10-year survival 60-80%). More than twice as many patients are listed for OLT relative to organs available, illustrating a compelling need to explore alternative treatment strategies for metabolic liver disease. We have recently developed a novel therapeutic strategy called metabolic pathway reprogramming. The concept rests on deletion of a critical metabolic gene in a disease- associated pathway, causing the metabolic pathway to be rerouted resulting in a benign disease phenotype. As a proof-of-principle, we focus on hereditary tyrosinemia type I (HT-1), which is caused by mutations of the fumarylacetoacetate gene (FAH). For many years, HT-1 patients have been treated with nitisinone, a drug that inhibits hydroxyphenylpyruvate dioxigenase (HPD), a gene upstream of FAH, and leads to accumulation of less toxic, excretable catabolites similar to the comparatively benign tyrosinemia type III (HT-III). We hypothesize that metabolic pathway reprogramming via somatic HPD gene deletion is an alternative to OLT for HT-1 patients and superior to the current pharmacological approach. We tested the concept of metabolic pathway reprogramming for HT-1 in a short-term (3 months) experiment using CRISPR/Cas9 genome editing and hydrodynamic tail vein injections (Pankowicz et al. Nat Commun.). While our approach was successful in mice, there are three major roadblocks for clinical translation; the long-term consequences of this therapy, the gene delivery method and the translation of this sequence specific therapy into the human setting. We propose to investigate these major roadblocks in the murine model of HT-1 and human liver chimeric mice utilizing a gene therapy approach with Adeno-Associated Virus (AAV) (Aim 1a). We will determine long-term benefit and risk of Hpd deletion by AAV in tyrosinemic mice over the state of the art therapy with nitisinone (Aim 1b), as well as determine efficiency and risk of such a therapy in humanized mice (Aim 2). Successful execution of this proposal will validate therapeutic applications of metabolic pathway reprogramming in primary human cells and has the potential to establish a new therapeutic paradigm for metabolic liver disease.